I recently purchased a vintage Fisher 500B receiver. Trying to connect 12 gauge speaker wire to the terminals has been difficult. Interestingly the, Fisher owners manual recommends 12 gauge "zip cord" for use as a speaker cable, but the terminals on the back of the receiver really don't accommodate wiring that thick. When the 12 gauge cord is wrapped around the terminal it almost touches other terminals due the the thickness of the wire.

12 gauge isn't zip cord, it's house wiring! Zip cord used for speakers is typically 18 ga and it works fine. People sometimes use 16 ga for longer runs.

I'd suggest using Sta-Kon terminals. The yellow ones are for 12ga wire but I think the smallest spade size is for a #10 screw, which may be too big. You may have to splice short lengths of 18 ga to your 12 ga wires.

PS: I was so skeptical about the 12 ga recommendation, I downloaded the Fisher 500B receiver manual. The recommendation is for "ordinary power cord". That would be 18 ga lamp cord.